Methods and compositions for ionizing radiation shielding

ABSTRACT

The radiation shielding composition and method of the present invention relate to a conformal coating material composed of a matrix of densely packed radiation shielding particles, which are disbursed within a binder. The shielding composition is applied to objects to be protected such as integrated circuits, or to packages therefor, as well as for protecting animals including humans from unwanted exposure to radiation in outer space or other environments.

TECHNICAL FIELD

[0001] The present invention relates in general to a radiation shieldingcoating composition and a method of making and using it. The inventionmore particularly relates to compositions and methods for shieldingmicroelectronic devices and other objects and animals, with aradiation-hardened light weight coating for withstanding the radiationhazards found in the space environment, as well as other less hazardousenvironments.

BACKGROUND ART

[0002] Many of today's commercial integrated circuit (IC) devices andmulti-chip modules (MCM) cannot be utilized in deep space and earthorbiting applications because of total dose radiation induced damage.Commercial IC devices are developed and manufactured for computer andmass market applications and are not designed to withstand the effectsof the natural space environment. The radiation effects include solarflares, galactic cosmic radiation and the Van Allen trapped electron andproton belts or man-made radiation induced events (neutrons and gammaradiation).

[0003] Typical commercial silicon integrated circuits fail to operatewhen exposed to total doses of two to fifteen kilorads(Si). Commonmethods used to prevent radiation degradation in performance are: 1) todesign special radiation tolerant die, 2) to shield the entire componentand board assembly, or 3) shield the individual component. There areweight, cost and time-to-market penalties depending on the method. Forexample, specially designed radiation tolerant die are time consumingand expensive to produce, since the part must be redesigned toincorporate radiation hardening techniques. Examples of such methodsinclude U.S. Pat. Nos. 3,933,530; 4,014,772; 4,148,049; 4,313,7684;4,402,002; 4,675,978; 4,825,278; 4,833,334; 4,903,108; 5,001,528;5,006,479; 5,024,965; 5,140,390; 5,220,192; and 5,324,952, each of whichpatent is incorporated herein by reference. Reference may also be madeto Japan patent 62-125651, Jun. 6, 1987, and articles entitled “Effectsof Material and/or Structure on Shielding of Electronic Devices,” R.Mangeret, T. Carriere, J. Beacour, T. M. Jordan, IEEE 1996; and “Novice,a Radiation Transport/Shielding Code”, T. M. Jordan, E.M.P. ConsultantsReport, January 1960, the Japan patent and such articles beingincorporated herein by reference.

[0004] Such techniques delay the time to market the products. As aresult, these conventional radiation hardened devices are usually two tothree generations behind the current commercial technological advancesin both size and capabilities. There are additional penalties in limitedmarketability and demand, and hence low volume productions of the dieresult. Consequently, such methods produce a more expensive product,which is technologically behind the commercially availablemicroelectronics, with slower speed and less capability. Additionally,because of the limited market for these products, they are frequentlynot available at all.

[0005] Such radiation shielding methods involve using metal shieldingexternal to the package. Shielding by other mechanical or electricalelements complicates the platform design, often requiring complex threedimensional modeling of the design.

[0006] Another attempt at shielding includes disposing a small shield onthe surface of the package. Such a technique does not provide effectivethree-dimensional shielding protection. Additionally, the small externalshield is generally thermally mismatched to the package, and increasesthe size and weight of the package.

[0007] Examples of system level shielding are disclosed in U.S. Pat.Nos. 4,833,334 and 5,324,952, which are incorporated by reference as iffully set forth herein. The U.S. Pat. No. 4,833,334 discloses the use ofa protective box to house sensitive electronic components. The box ispartially composed of a high atomic weight material to shieldeffectively against x-rays. However this approach has the seriousdisadvantage of adding substantial bulk and weight to electronic circuitassemblies protected in this manner. Moreover, it would be expensive toprovide this type of protection to individual integrated circuits asmanufacturing custom boxes for each circuit configuration would becostly.

[0008] The method of shielding material on the outside of the package isknown as spot shielding. Such a technique is disclosed in Japanesepatent publication 62-125651, published Jun. 6, 1987, which isincorporated by reference as if fully set forth herein. This patentdescribes a spot shielded semiconductor device which utilizes a doublelayered shield film to serve as a sealing cover on an upper surface of asemiconductor package. Another double layered shield film is attached toa lower surface of the package. However, space qualified microelectronicparts must be capable of withstanding the enormous forces exerted duringacceleration periods during space travel. The external shields aresubject to tearing or prying off from the sealing cover. The use of adouble layer shield film only slightly reduces the weight of thepackage, but increases the size of the package unnecessarily. Also, thinfilms are generally only effective at shielding electromagneticinterference (EMI) radiation and are ineffective at shielding ionizingradiation found in space. Examples of this type of EMI or EMF shieldingdevices include devices disclosed in U.S. Pat. Nos. 4,266,239;4,823,523; and 4,868,716, which are incorporated herein by reference.

[0009] The significant disadvantage of the spot shielding methodincludes an increase in weight and thickness of the device, and anincrease in exposure of the semiconductor to side angle radiation due tothe shielding being spaced apart from the semiconductor.

[0010] A far superior method of shielding involves using an integratedshield, where the package itself is the shield. For example, referencemay be made to said U.S. patent application Ser. No. 08/221,506, filedApr. 1, 1994, entitled “RADIATION SHIELDING OF INTEGRATED CIRCUITS ANDMULTI-CHIP MODULES IN CERAMIC AND METAL PACKAGES,” now U.S. Pat. No.______, which is incorporated herein by reference. The material in thepackage and the package design is optimized for the natural spaceradiation environment.

[0011] Many conventional microcircuits are only available in prepackagedform, or the die is already mounted onto the circuit board. Therefore,it would be highly desirable to have technique and shieldingcompositions for shielding parts already packaged or mounted on acircuit board, or in bare IC die form. Such compositions should berelatively inexpensive to manufacture and use, and are compact in size.In this regard, such new and improved techniques should be veryconvenient to employ in a highly effective manner, and yet be relativelyinexpensive to manufacture.

SUMMARY OF THE INVENTION

[0012] The principal object of the present invention is to provide a newand improved composition and method of radiation shielding in outerspace or other environments, whereby such shielding compositions andmethods are highly effective and relatively inexpensive.

[0013] Another object of the present invention is to provide such a newand improved method and composition, wherein the radiation tolerance ofthe bare die to be shielded is greatly improved, and the shielding isprovided in all axial directions.

[0014] A further object of the present invention is to provide such anew and improved method and composition, wherein satellite designers canutilize current generation IC technological advances, while improvingdelivery time.

[0015] A still further object of the present invention is to providesuch a new and improved method and composition, wherein IC devices canbe supplied relatively inexpensively due to the use of commerciallyavailable dies at current market prices without undue weight, excessiveor bulky sizes or other undesirable or unwanted design requirements.

[0016] Yet another object of the present invention is to provide such anew and improved composition and method of radiation shielding forprotecting other objects or animals from unwanted radiation.

[0017] Briefly, the above and further objects of the present inventionare realized by providing shielding compositions and methods which arerelatively inexpensive to use and highly effective in outer space andother environments.

[0018] The radiation shielding composition and method of the presentinvention relate to a conformal coating material composed of a matrix ofdensely packed radiation shielding particles, which are disbursed withina binder. The shielding composition is applied to objects to beprotected such as integrated circuits, or to packages therefor, as wellas for protecting animals including humans from unwanted exposure toradiation in outer space or other environments.

[0019] The inventive radiation shielding composition including thedensely filled conformal coating material is used for commerciallyavailable integrated circuits or grouping of circuits, to protectagainst natural and man-made radiation hazards of the spacecraftenvironment, whether in earth orbit, geostationary, or deep spaceprobes. The inventive composition and methods are provided to facilitatethe design and manufacture of microelectronics, and to coat externallythe microelectronics with the inventive shielding composition to improveradiation tolerance to natural space radiation.

[0020] The inventive shielding composition, in one form of theinvention, includes a fabric and a flexible binder, used to shieldanimals including humans in space or in other environments. As humansprolong their stay in space, the risks from increased exposure toionizing radiation become more of a concern. The conventional method ofshielding using lead has two major disadvantages. Lead is highly toxic,which is a disadvantage in both manufacture and use. Lead is alsorelatively less dense. With the inventive composition, the sameequivalent shielding can be obtained with a thinner high Z material suchas tungsten. By using a denser material, a thinner shield can beconstructed, making movement relatively easier. Since sources ofradiation are not limited to space, this same material has utility toshield humans or other animals from radiation sources on earth.

[0021] The limiting factor is weight, and the energy and species ofradiation. Thin densely packed shields are not very effective on highenergy electromagnetic radiation such as gamma rays, and high energyneutrons.

[0022] Additionally, the inventive conformal coating composition andmethod are useful as a radiation shielding gasket between enclosures.There are many radiation shielding utilities for the inventivecompositions and methods, depending on the choice of the bindermaterial.

[0023] The present inventive methods and compositions contemplate usingboth plastic or ceramic packaged microelectronic devices, as well asunpackaged die and encapsulating or coating the outer surface of thedevice to provide shielding as required for the anticipated radiationenvironment. Since fluences of species and energy ranges of radiationvary in space, and since the optimal shielding varies depending on thespecies of radiation, the coating substance or material can be optimallytailored based on the anticipated radiation that irradiates the part tobe protected. In all applications, the particles impregnated within theconformal coating substance are designed to achieve the highest tapdensity possible for the application.

[0024] The present inventive method preferably includescalculating/modeling the anticipated radiation spectrum, the requiredamount of shielding, as well as multiple layers of both high Z and low Zshielding material. The inventive conformal coating substance ormaterial is then designed to meet that requirement. For a standardGeosynchronous Orbit, the optimum shielding entails a conformal coatinghaving three layers; namely, a high Z layer sandwiched between two low Zlayers. For marking and hermiticity, a layer of smooth unimpregnatedcoating material is applied to the top layer.

[0025] For integrated circuit devices that have already been packaged,the inventive conformal coating material can be applied in variousmanners. These include, but are not restricted to, the followinginventive methods. One method relates to using a low pressure (or highpressure depending on the package strength and susceptibility) injectionmold. The coating material is injected into a mold containing thepackaged part. Another method involves “globbing” or putting a viscousconformal coating over a packaged part. The part can be disposed withina mold, or elsewhere when the shielding composition is applied. Anothermethod involves spraying or painting on the coating composition. Theoptimum method is to coat all sides of the part uniformly with theshielding composition to shield all sides equally from isotropicradiation, and especially when the direction of the source of radiationis not known.

[0026] For integrated circuits already attached to a board, either in abare die form or with an existing coating, the coating is applied with amold, by “globbing” the composition on, by spraying or painting. Toshield the top and bottom sides of the die uniformly, the bottom of theboard preferably is also shielded with the inventive conformal shieldingcomposition.

[0027] For multi-chip modules (MCMs) where there are multiple integratedcircuits within a single package, the inventive conformal coatingcomposition is applied in a similar manner as in the monolithic packagedintegrated circuit. Similarly, when there are multiple bare integratedcircuits, the inventive conformal coating composition is applied in asimilar manner as with the single bare integrated circuit, wherein thecoating composition is applied to the entire area covered by the devicesto be shielded.

[0028] For system or boxes containing board level products requiringadditional shielding, the inventive conformal coating composition canalso be applied to any box or device to be shielded from ionizingradiation. In this manner, with the use of a flexible binder materialsuch as latex, a gasket can be made for sealing two objects, wherein theinventive gasket material also provides a radiation shielding function.

[0029] Because of the flexibility of the inventive shieldingcomposition, radiation shielding can be achieved easily and relativelyinexpensively for applications that were either previously considered tobe excessively expensive or difficult to shield.

[0030] For human radiation protection, the inventive compositionconformal coating include a latex or similar flexible binder. To enhancethe mechanical strength properties, a fabric material is added andcombined with the binder. In this form of the invention, a high Zmaterial, which is dense and nontoxic, can be inserted within the layersof clothing material to add extra protection for the wearer fromunwanted radiation. Because of weight considerations, the optimalshielding can be obtained in the weightless environment of space.Lighter, thinner material is used for gravity constrained environments.Additionally, the impregnating particles can be tailored for the type ofradiation to be encountered, enabling optimal use of space and weight ofthe material.

BRIEF DESCRIPTION OF DRAWINGS

[0031] The above mentioned and other objects and features of thisinvention and the manner of attaining them will become apparent, and theinvention itself will be best understood by reference to the followingdescription of the embodiment of the invention in conjunction with theaccompanying drawings, wherein:

[0032]FIG. 1 is a diagrammatic sectional side view of a prior art spotshielded prepackaged integrated circuit;

[0033]FIG. 2A is a diagrammatic sectional side view of a conventionalunshielded commercial package assembly;

[0034]FIG. 2B is a diagrammatic top view of the package assembly of FIG.2A;

[0035]FIG. 3 is a diagrammatic sectional side view of a prior art modulewith multiple integrated circuit devices shielded therewithin;

[0036]FIG. 4 is a flow chart illustrating a radiation shielding methodaccording to the present invention;

[0037]FIG. 5 is a graph of a typical total dose versus depth curve,useful in understanding the present invention;

[0038]FIG. 6 is a diagrammatic sectional side view of a shieldingcomposition applied to a conventional package in accordance with thepresent invention;

[0039]FIG. 7 is a diagrammatic sectional side view of a shieldingcomposition applied to a conventional chip-on board in accordance withthe present invention;

[0040]FIG. 8 is a diagrammatic sectional side view of a multilayeredconformal coating of shielding composition applied to a conventionalintegrated circuit package in accordance with the present invention; and

[0041]FIG. 9 is a pictorial, partially diagrammatic fragmentary view ofa shielding composition used for animal radiation shielding inaccordance with the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

[0042] The following description is of the best mode presentlycontemplated for practicing the invention. This description is not to betaken in a limiting sense, but is made merely for the purpose ofdescribing the general principles of the invention. The scope of theinvention should be ascertained with reference to the issued claims. Inthe description that follows, like numerals or reference characters willbe used to refer to like parts or elements throughout.

[0043] Referring now to the drawings, and more particularly to FIGS. 1,2A and 2B, there is shown a commonly used conventional microelectronicpackage 10, which is a plastic package. FIG. 1 illustrates the package10 with a spot shield attached. The packages are comprised of a die 20,which is composed of silicon or other semiconductor base. The die isgenerally attached to a die attach pad 18 for support. The die is thenbonded with multiple lead wires 22, 24 to a lead frame with multipleleads 15, 16. This entire assembly is encased within a package 13composed of suitable plastic material or other material such as ceramic.

[0044] If thermal conductivity properties are important considerations,other materials such as ceramics are used, as shown in FIG. 3, these aremore difficult to work with and can be conducting, necessitating aninsulating feed through 25 to cover the leads 15, 16.

[0045] A conventional method for shielding these packages is shown inFIG. 1, where a pair of shielding plates 30 and 31, usually made of ahigh Z material such as tantalum, is attached to the top and bottomportions of the package 13 respectively by a suitable adhesive (notshown).

[0046] As shown in FIG. 3, another prior art technique relates to theuse of integrated shielding technology, where the package itself, ispart of the shielding. FIG. 3 shows the integrated shielding package 310that also incorporates multiple die 320 and 321. The multiple die 320and 321 on a die attach pad 318 employ multiple lead wires 322 and 324,together with a lead frame with multiple leads 315 and 316 and aninsulating feed through 325, for a package 313. This type of package iscalled an MCM or Hybrid package. With multiple die within the package,the density of functions increases, while the overall weight required toaccomplish the task is reduced. This type of packaging requires basemembers 340 and 341, which can be made of various shielding materials.For ionizing radiation, high Z materials can be used, enabling thepackage itself to become the radiation shielding.

[0047] As shown in FIG. 4, the inventive method includes, as indicatedin box 100, determining the inherent radiation tolerance of the die tobe shielded. This test can be accomplished by a Cobalt-60 source orother penetrating irradiation source. Without the knowledge of what theinherent radiation tolerance is for the individual semiconductor device,the designer does not know how much or whether shielding is necessary.

[0048] The next step as indicated at 102 involves determining theradiation spectrum and dose depth curve of the particular mission orradiation requirement of the application. For orbits around the earth,this is calculated using conventional radiation transport codes inconjunction with conventional radiation spectrum tables. The dose depthcurve is generally represented as a total radiation dose versusthickness of equivalent aluminum shielding as shown in FIG. 5. Althoughnot preferred, steps indicated at 100 and 102 can be omitted if theapplication is unknown and the designer desires only to enhance whateverthe radiation tolerance of the integrated circuit to be protected.

[0049] Knowing the inherent radiation tolerance of the integratedcircuit device, as indicated at 100 and the dose depth curve asindicated at 102, the amount of shielding required can be determined tobring the integrated circuit device within tolerance as indicated at104.

[0050] Knowing the spectrum of radiation for the application, thelayering of the inventive shielding material is tailored as hereinafterdescribed in greater detail with reference to FIG. 8. High Z material ismore effective at stopping electrons and Bremsstrahlung radiation, andless effective in stopping protons. Low Z material conversely is moreeffective at stopping protons and less effective at stopping electronsand Bremsstrahlung radiation.

[0051] The next step, as indicated at 106, requires determining the formof the integrated circuit. For a prepackaged part, the amount ofshielding is limited by the lead length on the bottom of the device,unless extenders are used. The most appropriate method of application ofthe inventive shielding composition is then determined as indicated at108. The part is coated in a mold (not shown), using a dam (not shown),and the coating can be globbed, sprayed, injected or painted on. For diethat are already mounted on the board (not shown), the methods mentionedabove are effective, but to insure uniform radiation shielding, thebottom of the board underneath the part is also coated with the samethickness of the inventive shielding composition. The coating materialis applied as indicated at 110 and then allowed to cure as indicated at111. Temporary extenders are preferably used to provide thorough wettingthroughout the binder. As an example, a preferred extender for epoxy isa high boiling point ketone.

[0052] Additionally, by adjusting the properties of the binder, the bulkelectrical properties of the shield composition is adjusted to be eitherinsulating or conductive.

[0053] Upon completion of coating the parts, testing is then performedelectrically and mechanically, as indicated generally at 112. For spaceapplications, the parts require space qualification testing.

[0054] There are various different methods of application of theinventive shielding composition as contemplated by the invention and asindicated in FIGS. 5, 6, 7 and 8. However, the following examples areintended to be representative and not all inclusive of the possibleapplication methods falling within the scope of the present invention.

[0055] Referring now to FIG. 6, a coating method of the presentinvention is illustrated for a die 600 attached to a substrate 604. Itshould be understood that a multiple die device, such as the one shownin FIG. 3, may also be protected as will become apparent to thoseskilled in the art.

[0056] The die is wire bonded at 606 and at 607 to lead frame devices602 and 603, respectively, to complete electrical connections betweenthe die and systems (not shown) outside of the package. A radiationshielding conformal coating composition is applied to the outside of thepackage 610. The package can then be applied to a board 615 or any otherattachment system by any suitable conventional technique.

[0057] The radiation shielding conformal coating composition 610 isapplied uniformly on the outer surface of the package to insure uniformradiation protection in accordance with the present invention. Thecoating can be applied by injection molding, mold casting, spraying,globbing or brushing the material onto the part to be protected.

[0058] Referring now to FIG. 7, another method of application accordingto the invention includes applying the radiation shielding conformalcoating composition generally indicated at 709 to an integrated circuitdevice 700 previously attached to a board 730. The board 730 may haveother devices such as a pair of devices 701 and 702 not requiringprotection. The device 700 is attached to the board via wire bonds 720and 721. The radiation shielding conformal coating composition 709 isthen applied both on top of the device 700 at 711 and directlyunderneath the device 77 at 710 on the board 730.

[0059] An area greater than the size of the device 700 is covered withradiation shielding conformal coating composition 710 on the bottom ofthe board 730. This is required to insure that the entire integratedcircuit device is protected from radiation.

[0060] The radiation shielding conformal coating composition 709 isapplied by the same method as described in connection with the inventivemethod of FIG. 6.

[0061] Referring now to FIG. 8, to enhance radiation shieldingperformance, multiple layers of the inventive radiation shieldingconformal coating composition are applied. Using conventional codes suchas NOVICE, different shielding layering are developed for each type oforbit. An optimum shielding geometry for a Geosynchronous Orbit is shownin FIG. 8.

[0062] As shown in FIG. 8, in accordance with the present invention, adie 800 having an integrated circuit package 804 containing lead framedevices 802 and 803 is encased within a multiple layer radiationshielding composition generally indicated at 820, prior to mounting theshielded die to a board or substrate 815. The multiple layer shieldingcomposition 820 comprises a layer of high Z particles 811 interposedbetween a pair of outer and inner layers of low Z particles 810 and 812.The low Z layer 812 is applied directly to the outer surface of the die800 in accordance with the method described in connection with FIG. 6.Thereafter, the intermediate high Z layer 811 is then applied to theouter surface of the inner low Z layer 812.

[0063] The outer low Z layer 810 is then applied to the outer surface ofthe intermediate high Z layer 811 to complete the shielding protectionfor the die 800. The shielded die 800 is then connected electrically andmounted to the board 815 by conventional techniques.

[0064] The high Z material is effective in stopping electrons andBremsstrahlung radiation, while the low Z material is more effective instopping protons. A Geosynchronous orbit is dominated by trappedelectrons, so it is preferable that the intermediate high Z layer 811,is thicker than the other two low Z layers. It will become apparent tothose skilled in the art that the multiple layer coating method of thepresent invention can be used in connection with the protection of manydifferent types and kinds of integrated circuit devices and the like.Additionally, the coating method can be applied by any method including,but not limited to, those described in connection with the method ofFIG. 6.

[0065] Referring to FIG. 9, there is shown a flexible shieldingmaterial, which is composed according to the present invention. Thematerial 900 contains the inventive radiation shielding composition, andis flexible and pliable to serve as clothing for humans or gasketmaterial for parts (not shown). The conformal coating material 900includes a flexible binder such as latex. The material 900 isimpregnated with a fabric such as a cloth woven material 910 forstrength. The cloth material can be composed of conventional materialssuch as cotton or polyester. For extra strength, nonwoven fabric such asKevlar or Teflon material can be used for the fabric.

[0066] Considering now the inventive radiation shielding compositionforming a part of the foregoing inventive methods and materials, thefollowing examples of shielding compositions are given to aid inunderstanding the invention, but it is to be understood that theparticular procedures, conditions and materials of these examples arenot intended as limitations of the present invention.

EXAMPLE I

[0067] 10.0 parts by weight high tap density tungsten powder 0.15 partby weight premixed epoxy up to 0.50 part by weight ketone

[0068] The tungsten powder serves as a high Z material for radiationshielding purposes. The epoxy serves as a binder to help adhere thecomposition to a surface, and the ketone is added as an extender.

[0069] To formulate the inventive composition, the ingredients ofExample I are mixed thoroughly, and then the mixture is applied to apart. The applied mixture is in the form of a paste, and is heatedslowly at a suitable low temperature such as 40° C. for about one hourto remove a substantial portion of the ketone extender withoutdisrupting the integrity of the packed tungsten powder. The mixture isthen heated at about 60° C. for about 16 hours to retain the stabilityof the composition. The temperature is then increased to about 150° C.for an additional period of time of about 0.5 hours. The resultingmixture has the desired consistency of a paste, and retains itsstability due to the foregoing multiple heating phases.

EXAMPLE II

[0070] In general, the ingredients of the present Example can beadjusted to accommodate variations in the foregoing described inventivemethods and applications.

[0071] The shielding powder can be any suitable high Z radiationshielding powder such as osmium, iridium, platinum, tantalum and gold.In general, any high Z material may be employed having an atomic numberof 50 and above. More preferably, the range of atomic numbers can bebetween 60 and 100, inclusive. The most preferred range of atomicnumbers is between 73 and 79, inclusive.

[0072] The shielding powder can also be a low Z material, such as theone mentioned in connection with the description of the inventive methodof FIG. 8. The low Z shielding powder is preferably selected from thegroup consisting of copper, nickel, carbon, titanium, chromium, cobalt,boron, silicon, iron and nitrogen. In general, any suitable low Zmaterial may be employed having an atomic number of 30 and below, butthe most preferred group of low Z materials is selected from the groupconsisting of copper, nickel, carbon, iron, titanium, silicon andnitrogen.

[0073] In general, the shielding powder can be any suitable materialcomposed of a matrix of densely packed shielding particles. Thepreferred material is tungsten (Example 1) having a packing density ofat least 150 grn per cubic inch.

[0074] There can be between about 0.10 and about 0.50 parts by weight ofa binder in the form of a suitable resin. The binder can be a urethane.The exact quantity of the binder determines the final density andstrength of the shielding afforded by the inventive composition. A morepreferred range of the binder is between about 0.13 and about 0.30.

[0075] Also, in general, the extender assures complete wetting of thepowders and adjusts the viscosity of the paste to suit the applicationmethod.

EXAMPLE III

[0076] 10.0 parts by weight high tap density tungsten powder 0.15 partby weight premixed epoxy up to 0.50 part by weight latex

[0077] This example of the inventive material may be used for the methoddescribed in connection with FIG. 9, wherein a fabric may be embeddedtherein for reinforcing purposes. Any suitable elastomer may be employedfor the latex.

[0078] As shown and described in the accompanying provisional patentapplication in Appendix A, there is described and shown a further andmore detailed disclosure of the inventive methods and compositions. Inthe Appendix A, the inventive radiation shielding composition isidentified by the trademark “RADCOAT.”

[0079] While particular embodiments of the present invention have beendisclosed, it is to be understood that various different modificationsare possible and are contemplated within the true spirit and scope ofthe appended claims. There is no intention, therefore, of limitations tothe exact abstract or disclosure herein presented.

What is claimed is:
 1. A conformal coating composition, comprising: anamount of high Z shielding particles densely packed at a concentrationgreater than 60% in a first binder; and an amount of low Z shieldingparticles densely packed at a concentration greater than 60% in a secondbinder, wherein said amount of high Z shielding particles and saidamount of low Z shielding particles are sufficient to shield an objectfrom receiving an amount of radiation greater than a total dosetolerance of said object.
 2. The composition of claim 1, wherein saidobject is an integrated circuit.
 3. The composition of claim 1, whereinsaid high Z shielding particles are selected from the group consistingof tungsten, osmium, iridium, platinum, tantalum, and gold.
 4. Thecomposition of claim 1, wherein said low Z shielding particles areselected from the group consisting of copper, nickel, carbon, titanium,chromium, cobalt, boron, silicon, iron, and nitrogen.
 5. The compositionof claim 1, wherein said first and second binders are selected from thegroup consisting of latex, urethane, and epoxy.
 6. A flexible shieldingcomposition, comprising: a fabric; and an amount of high Z shieldingparticles densely packed at a concentration greater than 60% in a firstflexible binder impregnated into said fabric; and an amount of low Zshielding particles densely packed at a concentration greater than 60%in a second flexible binder impregnated into said fabric, wherein saidamount of high Z shielding particles and said amount of low Z shieldingparticles are sufficient to shield an object from receiving an amount ofradiation greater than a total dose tolerance of said object.
 7. Thecomposition of claim 6, wherein said flexible shielding composition isclothing for shielding a living object.
 8. The composition of claim 6,wherein said flexible shielding composition is gasket material.
 9. Thecomposition of claim 6, wherein said fabric is selected from the groupconsisting of cotton, polyester, Kevlar, and Teflon.
 10. The compositionof claim 6, wherein said high Z shielding particles are selected fromthe group consisting of tungsten, osmium, iridium, platinum, tantalum,and gold.
 11. The composition of claim 6, wherein said low Z shieldingparticles are selected from the group consisting of copper, nickel,carbon, titanium, chromium, cobalt, boron, silicon, iron, and nitrogen.12. The composition of claim 6, wherein said first and second bindersare selected from the group consisting of latex, urethane, and epoxy.13. A method of designing a shielding composition, comprising:determining the radiation tolerance of the object to be shielded; anddetermining the radiation requirement for the particular application;and determining the amount of said shielding composition required tobring said object within tolerance relative to said determined radiationtolerance of said object and said determined radiation requirement ofsaid application, wherein said shielding composition consists of anamount of high Z particles densely packed at a concentration greaterthan 60% in a first binder and an amount of low Z particles denselypacked at a concentration greater than 60% in a second binder.
 14. Themethod of claim 13, wherein said object is an integrated circuit. 15.The method of claim 13, wherein said object is a living thing.
 16. Themethod of claim 13, wherein said high Z shielding particles are selectedfrom the group consisting of tungsten, osmium, iridium, platinum,tantalum, and gold.
 17. The method of claim 13, wherein said low Zshielding particles are selected from the group consisting of copper,nickel, carbon, titanium, chromium, cobalt, boron, silicon, iron, andnitrogen.
 18. The method of claim 13, wherein said first and secondbinders are selected from the group consisting of latex, urethane, andepoxy.
 19. A method of designing a shielding composition, comprising:estimating the amount of shielding composition required to bring anobject within a tolerance, wherein said shielding composition consistsof an amount of high Z particles densely packed at a concentrationgreater than 60% in a first binder and an amount of low Z particlesdensely packed at a concentration greater than 60% in a second binder.20. The method of claim 19, wherein said object is an integratedcircuit.
 21. The method of claim 19, wherein said object is a livingthing.
 22. The method of claim 19, wherein said high Z shieldingparticles are selected from the group consisting of tungsten, osmium,iridium, platinum, tantalum, and gold.
 23. The method of claim 19,wherein said low Z shielding particles are selected from the groupconsisting of copper, nickel, carbon, titanium, chromium, cobalt, boron,silicon, iron, and nitrogen.
 24. The method of claim 19, wherein saidfirst and second binders are selected from the group consisting oflatex, urethane, and epoxy.
 25. A method of shielding an object,comprising: applying a conformal coating composition composed of anamount of high Z shielding particles densely packed at a concentrationgreater than 60% in a first binder and an amount of low Z shieldingparticles densely packed at a concentration greater than 60% in a secondbinder, wherein said amount of shielding particles are sufficient toshield an object from receiving an amount of radiation greater than atotal dose tolerance of said object.
 26. The method of claim 25, whereinsaid object is an integrated circuit.
 27. The method of claim 25,wherein said object is a living thing.
 28. The method of claim 25,wherein said shielding composition is applied to the exterior of saidobject.
 29. The method of claim 25, wherein said shielding compositionis applied equally in all axial directions.
 30. The method of claim 25,wherein said high Z shielding particles are selected from the groupconsisting of tungsten, osmium, iridium, platinum, tantalum, and gold.31. The method of claim 25, wherein said low Z shielding particles areselected from the group consisting of copper, nickel, carbon, titanium,chromium, cobalt, boron, silicon, iron, and nitrogen.
 32. The method ofclaim 25, wherein said first and second binders are selected from thegroup consisting of latex, urethane, and epoxy.
 33. A method ofshielding an object, comprising: inserting said object into an injectionmold; and injecting a conformal coating composition composed of anamount of high Z shielding particles densely packed at a concentrationgreater than 60% in a first binder and an amount of low Z shieldingparticles densely packed at a concentration greater than 60% in a secondbinder into said injection mold containing said object, wherein saidamount of high Z shielding particles and said amount of low Z shieldingparticles are sufficient to shield said object from receiving an amountof radiation greater than a total dose tolerance of said object.
 34. Themethod of claim 33, wherein said object is an integrated circuit. 35.The method of claim 33, wherein said shielding composition is applied tothe exterior of said object.
 35. The method of claim 33, wherein saidshielding composition is applied equally in all axial directions. 36.The method of claim 33, wherein said high Z shielding particles areselected from the group consisting of tungsten, osmium, iridium,platinum, tantalum, and gold.
 37. The method of claim 33, wherein saidlow Z shielding particles are selected from the group consisting ofcopper, nickel, carbon, titanium, chromium, cobalt, boron, silicon,iron, and nitrogen.
 38. The method of claim 33, wherein said first andsecond binders are selected from the group consisting of latex,urethane, and epoxy.